# first method
### This error appears when you remove a template folder, which is still referenced in the database. The simplest way to fix it is, go to the database, and in table 'theme', find the theme name which does not exist in app/design/frontend/package.
### DELETE the reference to the missing theme from the table.

# Second method
### I have the same error Required parameter 'theme_dir' was not passed. I work with 2.1.0 from the beginning, not with updating. And I have one custom product attribute "style".
### The problem occurs when I try to save the new photo in the material. Without the image, the product saved normally.
### Fix with commenting the exception under the file:
### vendor/magento/framework/view/design/Fallbck/Rule/Simple.php
### It worked for me. But I think it's a hack.

# Finally
### I fixed it by commenting out the exception under vendor/magento/framework/View/Design/Fallback/Rule/Simple.php.

if (in_array($placeholder, $this->optionalParams)) {
    return [];
} else {
    return [];
    //throw new \InvalidArgumentException("Required parameter '{$placeholder}' was not passed");
}